While searching for add-ons, I saw that Firefox for Android has plugins for fallback fonts, e.g. for Khmer and Burmese (but not Polytonic Greek).
I tried them and Burmese and Khmer displayed in Firefox. No rootiing done.
So, technically it shouldn't be that hard to have a Polytonic Greek version with the Ubuntu font.

Wow - so much great info here! I am a newbie studying ancient Greek, but as an IT geek I have spent a lot of time in the last few years playing with classical Greek fonts on the PC and on Android, so here are my own offerings (forgive me where I repeat what other contributers have mentioned).

Keyman For Android (formally Keyman Pro) is now a completely free app. By setting the option to use as a "system-wide keyboard", you can type ancient Greek (among many other languages) successfully into almost any text app on an Android device. There are a few layout alternatives - my preferred is Greek, Ancient > Greek Polytonic. This reproduces all characters and diacritics for the student (with the possible exception of the letter eta with both a iota subscript plus circumflex ῇ - I have requested a fix by the developer). Keymen works well with both the on-screen keyboard or with an external bluetooth keyboard.

Installing a classical Greek font on the device. The Samsung Galaxy Tab S2 (and some other Galaxy tablets) allow you to change the system font under Settings > Display. To load a non-Samsung font, download the iFont Donate app for a couple of bucks which allows you to add other fonts to the system directory. In iFont, click on the tab My > My Font where you can attempt to install any .TTF font file that you have downloaded for yourself. My favorite Greek font is IFAO-Grec Unicode 2.0 http://www.ifao.egnet.net/publications/ ... d/polices/ as it has a real circumflex (not tilde) accent and big boldly rendered diacritics which are easy to read.

On my Sony C6603 Xperia phone, I have also installed IFAO Grec Unicode font as system default, also using iFont Donate app. However it is not possible to change the default font on all devices unless rooted, which it is in this case.

AnkiDroid flash card app. I have been using the amazing (free) Anki flash card software on my PC for several years. This can be synchronised to your Android device using the Ankidroid app (created by a separate developer). Ankidroid, in its settings, offers a handful of fonts including the default system font. However, by using the device's file manager app (or a download such as Solid Explorer) and adding a folder named "fonts" under Internal > Ankidroid, you can put in your own font of choice which then becomes available to display when working with your flash cards.

AnkiDroid flash card app. I have been using the amazing (free) Anki flash card software on my PC for several years. This can be synchronised to your Android device using the Ankidroid app (created by a separate developer). Ankidroid, in its settings, offers a handful of fonts including the default system font. However, by using the device's file manager app (or a download such as Solid Explorer) and adding a folder named "fonts" under Internal > Ankidroid, you can put in your own font of choice which then becomes available to display when working with your flash cards.

So I have been trying to set up classical greek alphabet on my tablet for some time now, downloaded Keyman and it has been working fine but when I tried to follow your advices about AnkiDroid quoted above, nothing happened - AnkiDroid still shows polytonic greek characters as squares or question marks or in form of empty space

I created a subfolder "Fonts" in Device>AnkiDroid and in that folder I installed IFAOGrec.ttf but the AnkiDroid ignores it completely

I really want my flashcards, created on PC using Keyman, to work on my tablet as well, so please, help me!

Meanwhile I finally set it up - you have to not only create a subfolder "fonts" and put the polytonic fonts inside but you also have to go into settings in AnkiDroid and set the chosen polytonic font as 1) Default font and as 2) Browser and editor font - now it is working like a charm!